Refactor storage of Topicmap State

Currently topicmap state (pan + zoom values) is stored as topics. In the face of DM5's immutable value storage this feels like overkill. It makes no sense to have hundreds and hundreds of x/y and zoom (floating point!) value topics in the database.

Topicmap state should be stored as DB-props instead.

(Note: a while ago topic coordinates were transformed from topics to DB-props already, but the topicmap state is not.)